1. Field of the Invention
The present invention is directed to a magnetic resonance imaging apparatus of the type suitable for undertaking diagnostic examinations of a human patient having a body axis extending along the x-axis of a rectangular coordinate system, with the body region to be examined being located between the pole pieces of a magnet forming a fundamental magnetic field extending in the direction of the z-axis, and in particular to an RF antenna for such an apparatus.
2. Description of the Prior Art
The use of magnetic resonance imaging device for producing tomograms, based on the principle of nuclear magnetic resonance, of a body slice, for conducting diagnostics of anatomical joints, and for portraying blood vessels are well known. In such devices, an image is constructed by computational or mensurational analysis of integral proton resonance signals from the spatial spin density, or the relaxation time distribution, of the examination subject. The examination subject, such as a human body, is introduced into a strong, uniform magnetic field, referred to as the fundamental or basic field, which aligns the nuclear spins in the examination subject. Gradient coils are provided which each generate a spatially differently oriented magnetic field. An RF antenna excites the nuclear spins in the examination subject, and transmits the resulting signal emitted by the excited nuclear spins to a receiver. The RF antenna is usually connected to a transmitter and to a receiver via matching capacitors as well as via a transmission/reception diplexer. Whereas the maximum transmission power in such a device is established by the load limit of the components, the maximum, mean transmission power is essentially limited by the level of heating which can be withstood by the patient.
As is known, circularly polarizing antennas require a low transmission power. These antennas have the advantage of essentially producing only field components effective for the nuclear magnetic resonance, for example the counter-clockwise field components. For example, such an antenna may consist of two linearly polarizing antenna systems arranged orthogonally relative to each other and connected to a transmitter and to a receiver via a 90.degree. directional coupler. The transmission signal which is fed to the antenna is supplied directly to one of the antenna systems, and is supplied with a 90.degree. phase shift to the other antenna system, so that a rotating field is generated which is effective for conducting nuclear magnetic resonance tomography. In the reception mode, the antenna represents two signal sources phase-shifted by 90.degree. and also represents two uncorrelated noise sources. The 90.degree. directional coupler supplies the in-phase sum of the signals to a receiver. Such an antenna is described in the Journal of Magnetic Resonance, Vol. 54 (1983) at pages 324-327.
Generally, the fundamental static magnetic field is generated in the direction of the a. c. axis of the patient. Such magnets are generally executed as solenoid magnets, which are superconducting for stronger magnetic fields above 0.5 T. It is also known to employ a fundamental field magnet in a magnetic resonance tomography apparatus wherein the fundamental field extends perpendicularly relative to the a. c. axis of the patient, i.e., in the direction of the z-axis of a rectangular coordinate system. Such a fundamental field magnet is disclosed in European Application 0 161 782.
The fundamental field magnet is provided with pole pieces which limit the imaging volume, and between which the uniform fundamental field is to be generated. The pole pieces may be connected to each other via the common yoke of a permanent magnet or an electromagnet, and thus form what is known as a C-magnet. Such a structure is disclosed in German OS 37 37 133.